1. Field of the Invention:
This invention relates to a joint mechanism in which several joints are connected in series so as to get a soft resiliency. More particularly, it relates to a joint mechanism which may be flexed in any direction and in which a highly precise positioning of the distal end of the joint mechanism may be obtained; high durability, softness and rigidity can be obtained; and which is preferable for arms or wrists of robots or manipulators, etc.
2. Description of the Prior Art:
As one example of a soft joint mechanism, a robot arm described in Japanese Laid-Open Patent Application No. 57-66890 is already known. This robot arm is made such that, as shown in FIG. 1(a), the closed ring-shaped links A, B, C . . . are linked like a chain. The adjoining links are connected by rotary sliding joints J in which an axial sliding and a swivelling can be performed simultaneously. Alternate links are swivellingly connected to each other by ball joints BJ arranged on a central axis of the joints. A fixed machine table M and the proximal link A are swivellingly connected by a ball joint BJ.sub.1, and the second link B is also connected to the fixed machine table M with a ball joint BJ.sub.2.
In the case of the above-described robot arm, the proximal link A is oscillated (by means not shown) in respect to the machine table M in a horizontal plane containing the link A around the ball joint BJ.sub.1. This oscillation causes all the links to swivel in the swivelling direction. That is, each of the joints undergoes a swivelling movement, so that an entire smooth swivelling movement may be provided and a soft arm may also be attained.
However, in the case of the above-described robot arm, when the link A is swivelled in a vertical plane crossing at a right angle with a horizontal plane containing the above link A, for example, by an angle .alpha..sub.1 around the ball joint BJ.sub.1, as shown in FIG. 1(b), a member connecting the link A to the link B is fixed to the link A at a right angle thereto, so that, after the swivelling operation, the link B has an inclination angle of .alpha..sub.1 in respect to a vertical plane Y.sub.1.
In turn, in the case of the link B, the link B swivels only by an angle .alpha..sub.2 due to a swivelling operation of the above link A around the ball joint BJ.sub.2 (placed at the fixed position). It can easily be seen that .alpha..sub.1 .noteq..alpha..sub.2, due to the different rotational centers of the links A, B. Along with this movement, the member X.sub.1 is also inclined at an angle of .alpha..sub.2 in respect to the vertical plane Y.sub.1 (to X.sub.1 ').
In turn, the member X.sub.1 is fixed at a right angle to the link A as described above, so that the member X.sub.1 must be inclined by an angle of .alpha..sub.1 in respect to the vertical plane Y.sub.1 due to the swivelling operation of the link A. Accordingly, the angle .alpha..sub.2 in respect to the vertical plane Y.sub.1 can not be obtained. That is, the member X.sub.1 can not maintain a right angle in respect to the upper and lower sides B.sub.1 and B.sub.2 of the link B. Accordingly, the member X.sub.1 can be inclined only by an angle of (.alpha..sub.2 -.alpha..sub.1) in respect to the sides B.sub.3 and B.sub.4, which are perpendicular to the sides B.sub.1 and B.sub.2.
However, since the link B is naturally composed of highly rigid material, the distance between the sides B.sub.1 and B.sub.2 forming the link B is kept constant, and the length of the member X.sub.1 is also kept constant. Furthermore, the distance between the rotary sliding joints J, J at both ends of the member may not be varied, so the member X.sub.1 is kept at a right angle in respect to the sides B.sub.1 and B.sub.2, and the member X.sub.1 may not be inclined only by an angle of (.alpha..sub.2 -.alpha..sub.1) in respect to the sides B.sub.3 and B.sub.4.
Therefore, in the case of the robot arm as described above, the folding direction is limited in a horizontal plane containing the link A shown in FIG. 1(a). That is, the arm may not be folded in a direction perpendicular to the former folding direction, and so this arm is not capable of being flexed in any direction.
This type of robot arm has the disadvantage that, if the arm is twisted by an application of external force, the swivelling function of the rotary sliding joint J is decreased, and a smooth swivelling movement may not be performed due to a so-called twisted condition.
That is, for example, assuming that the intermediate link B is taken out as shown in FIG. 2, if a twisting moment indicated by an arrow 1 is applied to the link B, the four sides B.sub.1 B.sub.2, B.sub.3 and B.sub.4 constituting the link B are displaced to B.sub.1 ', B.sub.2 ', B.sub.3 ' and B.sub.4 ' as viewed in the figure, the rotary sliding joints J.sub.1, J.sub.4 and J.sub.2, J.sub.3 fixed to the sides B.sub.1 and B.sub.2 are displaced to the positions J.sub.1 ', J.sub.4 ' and J.sub.2 ', J.sub.3 ', respectively, and the distances J.sub.1, J.sub.2 and J.sub.3, J.sub.4 between the upper and lower rotary sliding joints are displaced to J.sub.1 ', J.sub.2 ' and J.sub.3 ', J.sub.4 '.
In turn, this type of robot arm has the disadvantage that, since the upper and lower rotary sliding joints J.sub.1 and J.sub.2 are connected by the member X.sub.1 and similarly the upper and lower sliding joints J.sub.3 and J.sub.4 are connected by the member X.sub.2 such that their distances may not be varied as described above, an application of twisting moment to the link B as above may cause a force for prohibiting the sliding of each of the rotary sliding joints J.sub.1 to J.sub.4. At the same time, each of the rotary sliding joints J.sub.1 to J.sub.4 may be twisted in respect to each of the sides B.sub.1 and B.sub.2, resulting in the sliding function of each of the rotary sliding joints being diminished and the folding operation at each of the joints being made null.
Such disadvantages as above are based on the fact that this type of robot arm has the rotary sliding joint at a position displaced from the axis G of each of the links and has a connection between the upper and lower rotary sliding joints, for example, J.sub.1 and J.sub.2 with the member X.sub.1 etc. in such a way that the distance between them may be kept constant.
The robot arm described above has further disadvantages in that the above rotary sliding joint J is required in order to cause the fixing positions of the members X.sub.1 and X.sub.2 in respect to the sides B.sub.1 or B.sub.2 to be variable The rotary sliding joint J has in general a ball bearing therein, and an increased loading capacity of the ball bearing may require an application of a large-sized ball bearing. The rotary sliding joint J is, as shown in FIG. 1(a), placed at the outermost part of the joint mechanism, so that the application of a large-sized ball bearing causes the entire joint mechanism to be of a large size. Furthermore, the sides B.sub.1 and B.sub.2 pass through the ball bearing, so that the member X.sub.1 exhibits a narrow swivelling range in respect to the side B.sub.1 etc., and a large folding angle may not be obtained.
As described above, this type of robot arm shows additional disadvantages that, since the ball bearing as above may not be made large, the length S of the sides B.sub.1 and B.sub.2 in sliding contact with the ball bearing may not be made long. Correspondingly, a substantial looseness of the sides B.sub.1 and B.sub.2 in respect to the members X.sub.1 and X.sub.2 is produced, and an accurate precision of positioning may not be obtained.